Various types of recordable information recording mediums have been put into practice for recording video or audio data or storing personal computer data or the like. For example, CD mainly used for recording audio data or storing personal computer data and DVD used for recording video data or storing personal computer data have increasingly spread. Recently, BD (Blu-ray Disc) on which high vision video of high image quality including video of digital broadcasting can be recorded has been put on the market.
The above-described information such as video, audio or personal computer data is recorded on an information recording medium as user data. Specifically, the user data is provided with an error correction code and is modulated into a data sequence including recording marks and spaces having a prescribed range of length. The data sequence is recorded on a track of the information recording medium using a light beam. From the information recorded on the information recording medium, an analog reproduction signal is generated. The analog reproduction signal is specifically generated from reflected light which is obtained by irradiating the track with a light beam and includes information corresponding to the data sequence, namely, the recording marks and spaces. From the reproduction signal, a data sequence, which is a binary signal, is generated. The data sequence is decoded and then subjected with error correction. Thus, user data is obtained.
FIG. 1 shows various types of signals usable for forming a recording mark on an information recording medium. FIG. 1, part (a), shows a channel clock signal of a cycle Tw, which acts as a reference signal used for generating recording data. FIG. 1, part (b), shows an NRZI (Non Return to Zero Inverting) signal, which is a modulation code obtained by modulating information to be recorded.
In the case of, for example, BD, the NRZI signal is obtained by modulating information to be recorded using recording marks and spaces having a length of 2T (2×Tw) to 8T based on the cycle Tw as the reference. FIG. 1, part (b), shows a pattern of 2T mark-2T space-4T mark as a general example of a part of the NRZI signal.
FIG. 1, parts (c) and (d), respectively show a recording pulse sequence of recording laser generated based on the NRZI signal and a data sequence (recording mark sequence) formed on the information recording medium.
A recording mark of each length is formed by a recording pulse sequence including at least a first pulse (also referred to as a “leading pulse”). The recording mark further includes a last pulse and at least one middle pulse located between the first pulse and the last pulse, depending on the length of the recording mark. A pulse width of the first pulse, Ttop, and a pulse width of the last pulse, Tlp, are each set in accordance with the length of the recording mark. A pulse width of the middle pulse, Tmp, is always set to the same length regardless of the length of the recording mark.
A level of the recording pulse sequence, namely, a laser intensity is classified as a peak power Pp201 which provides a heating effect required for forming a recording mark, a bottom power Pb202 having a cooling effect, a cooling power Pc203, and a space power Ps204 which is a recording power in the space. The peak power Pp201, the bottom power Pb202, the cooling power Pc203 and the space power Ps204 are set with respect to an extinction level 205, detected when the laser light is turned off, as the reference level.
The bottom power Pb202 and the cooling power Pc203 are generally set to an equivalent recording power. However, the cooling power Pc203 may be set to a different value from the bottom power Pb202 in order to adjust the heat amount at an end of a recording mark. The space power Ps204 is generally set to a low recording power (for example, a recording power equivalent to a reproduction power or the bottom power) because it is not necessary to form a recording mark in a space. However, for a rewritable optical disc (for example, DVD-RAM or BD-RE), the existing recording mark needs to be erased to create a space. For a write once optical disc (for example, DVD-R or BD-R), a preheating power for creating the next recording mark may be occasionally provided. For these reasons, the space power Ps204 may be set to a relatively high recording power. Even in this case, the space power Ps204 is never set to be a higher value than that of the peak power Pp201.
The recording mark formed by irradiation with laser of a prescribed power depends on the characteristics of each information recording layer of the information recording medium. Therefore, the information recording medium has recorded thereon laser emission conditions for recording such as the laser power value, the pulse width and the like of the recording pulse sequence suitable to the information recording medium. By appropriately reproducing the laser power and the pulse width of the recording pulses recorded in the information recording medium and irradiating the information recording medium with appropriate laser light, a recording mark sequence can be formed.
However, the characteristics of each information recording layer of the information recording medium and the laser emission characteristics of the recording apparatus are varied for an individual information recording medium or an individual recording apparatus. The influence of heat is also varied in accordance with the environment of use. Thermal interference may be caused from an adjacent recording mark. For these reasons, at least each time a new information recording medium is mounted, the recording apparatus generally performs test recording to evaluate the obtained reproduction signal and to fine-tune the pulse shape of the recording laser based on the evaluation results, so that correct recording marks are formed. For example, for each length of recording mark, recording start position offset dTtop for adjusting the start position of the recording mark and the recording end position offset dTs for adjusting the end position of the recording mark are set, and these offset values are adjusted at the time of test recording.
The recording pulses included in the recording pulse sequence may have a mono-pulse waveform, an L-shaped pulse waveform or a castle-type pulse waveform as shown in FIG. 2, parts (a), (b) and (c) in addition to the above-described multi-pulse waveform. In general, with the mono-pulse waveform, as the recording mark is longer, the amount of accumulated heat increases. With the L-shaped pulse waveform, as the recording mark is longer, the amount of accumulated heat decreases. With the castle-type pulse waveform, the heat amount at the end of the recording mark is adjusted. With the multi-pulse waveform, the amount of accumulated heat is constant regardless of the length of the recording mark. In consideration of these, an appropriate waveform is selected in accordance with the layer characteristics of the information recording layer of the information recording medium, especially the characteristics of the accumulated heat.
Recently, as the display precision of video is raised, an information recording medium having a larger capacity is desired. In order to increase the recording density of the information recording medium, the recording marks used for recording information need to be smaller. However, as the recording marks become smaller, the shortest recording mark length is close to the limit of the optical resolution, and so the increase of the inter-symbol interference and the deterioration of the SNR (signal-to-noise ratio) become conspicuous. As a result, the leading edge or the trailing edge of the recording mark cannot be correctly detected, which makes it difficult to correctly decode the recorded information from the reproduction signal.
For this reason, for reproducing information from an information recording medium on which information is recorded with small recording marks, it has become popular to process the reproduction signal using a PRML (Partial Response Maximum Likelihood) system or the like. The PRML system is a combination technology of partial response (PR) and maximum likelihood decoding (ML), and estimates waveforms of the reproduction signal when known inter-symbol interference occurs and selects a most likely signal sequence from the estimated waveforms.
As the recording marks become small, thermal interference occurs. Specifically, the heat at the end of the recording mark is conducted through the space and influences the temperature rise at the start of the subsequent recording mark, or the heat at the start of the subsequent recording mark influences the cooling process at the end of the previous recording mark. When such thermal interference occurs, space compensation needs to be provided by test recording. Space compensation is to change the recording parameters (for example, dTtop) of the recording pulse in accordance with the length of the previous space or the subsequent space.
Patent Documents No. 1 and No. 2, for example, each describe a conventional method for controlling the recording pulse in consideration of the influence of the inter-symbol interference or thermal interference.
According to the method disclosed in Patent Document No. 1, a correct bit stream obtained by correct demodulation and an error bit stream with a maximum likelihood of error, which is generated as a result of one bit of the correct bit stream being shifted, are used to calculate an Euclidian distance between the reproduction signal and each of both bit streams. Thus, a reproduction signal adaptively equalized is evaluated, thereby detecting an edge shift direction and an edge shift amount of each pattern. Adaptive recording parameters classified by the length of the recording mark to be formed and the length of the space immediately previous or subsequent thereto are optimized in accordance with the edge shift direction and the edge shift amount corresponding to each pattern.
According to Patent Document No. 2, for an edge at which one bit is shifted from a correct bit stream and an incorrect bit stream, a difference between the amplitude value of an adaptively equalized reproduction signal and an expected amplitude value calculated in each stream is quantified. Thus, an edge shift direction and an edge shift amount are detected. Like in Patent Document No. 1, the adaptive recording parameters organized in a table by the length of the mark and the length of the space immediately previous or subsequent thereto are optimized in accordance with the edge shift direction and the edge shift amount corresponding to each pattern.
In Patent Documents No. 1 and No. 2, a reproduction signal is processed by a PR1221ML system. The recording pulse control disclosed in Patent Document No. 1 will be further described with reference to FIG. 3.
Information read from an information recording medium 1 is generated as an analog reproduction signal by an optical head 2. The analog reproduction signal is amplified and AC-coupled by a preamplifier 3, and then input to an AGC section 4. The AGC section 4 adjusts the amplitude such that the output from a waveform equalizer 5 on a later stage has a constant amplitude. The amplitude-adjusted analog reproduction signal is waveform-shaped by the waveform equalizer 5 and input to an A/D conversion section 6. The A/D conversion section 6 samples the analog reproduction signal in synchronization with a reproduction clock output from a PLL section 7. The PLL section 7 extracts the reproduction clock from a digital reproduction signal obtained by the sampling performed by the A/D conversion section 6.
The digital reproduction signal generated by the sampling performed by the A/D conversion section 6 is input to a PR equalization section 8. The PR equalization section 8 adjusts the frequency of the digital reproduction signal such that the frequency characteristic of the digital reproduction signal at the time of recording/reproduction is the characteristic assumed by a maximum likelihood decoding section 9 (for example, PR(1,2,2,1) equalization characteristic). The maximum likelihood decoding section 9 performs maximum likelihood decoding on the waveform-shaped digital reproduction signal output from the PR equalization section 8 to generate a binary signal. The reproduction signal processing technology provided by combining the PR equalization section 8 and the maximum likelihood decoding section 9 is the PRML system.
An edge shift detection section 10 receives the waveform-shaped digital reproduction signal output from the PR equalization section 8 and the binary signal output from the maximum likelihood decoding section 9. The edge shift detection section 10 distinguishes a state transfer from the binary signal, and finds the reliability of the decoding result from the distinguishing result and the branch metric. The edge shift detection section 10 also assigns the reliability for each of leading edge/trailing edge patterns of recording marks based on the binary signal and finds a shift of a recording compensation parameter from the optimal value (hereinafter, the shift will be referred to as the “edge shift”).
Test recording is performed using a data sequence having a prescribed recording pattern. An information recording control section 15 changes a recording parameter, the setting change of which is possible, in conformity to the information indicating that the setting change of the recording parameter is determined as being required based on the edge shift amount detected for each pattern. The recording parameters, the setting of which is changeable, are predetermined. Such recording parameters include, for example, the recording start position offset dTtop regarding the leading edge of a recording mark and the recording end position offset dTs regarding the trailing edge of a recording mark. The information recording control section 15 changes the recording parameter in accordance with the table of the recording parameters shown in FIG. 4. FIG. 4 shows recording parameters regarding the leading edge classified by the length of the recording mark and the length of the space immediately previous thereto, and recording parameters regarding the trailing edge classified by the length of the recording mark and the length of the space immediately subsequent thereto.
In FIG. 4, the symbols of recording mark M′(i), immediately previous space S(i−1) and immediately subsequent space S(i+1) are used in the time series of recording marks and spaces shown in FIG. 5. Symbol M represents a recording mark and symbol S represents a space. A position in the time series of an arbitrary recording mark or space is represented using symbol i.
The recording mark corresponding to the recording parameter shown in FIG. 4 is represented by M(i). As shown in FIG. 5, a space immediately previous to the recording mark M(i) is S(i−1), a recording mark further immediately previous is M(i−2), and a space still further immediately previous is S(i−3). A space immediately subsequent to the recording mark M(i) is S(i+1), a recording mark further immediately subsequent is M(i+2), and a space still further immediately subsequent is S(i+3).
The leading edge is located between the recording mark M(i) and the immediately previous space S(i−1). As shown in FIG. 4, the value of dTtop is classified by the pattern in accordance with a combination of the lengths thereof. For example, in the case where the length of the immediately previous space is 3T and the length of the recording mark is 4T, the pattern 3Ts4Tm is used. The trailing edge is located between the recording mark M(i) and the immediately subsequent space S(i+1). As shown in FIG. 4, the value of dTs is classified by the pattern in accordance with a combination of the lengths thereof. For example, in the case where the length of the recording mark is 3T and the length of the immediately subsequent space is 2T, the pattern 3Tm2Ts is used. As shown in FIG. 4, there are a total of 32 recording parameter values regarding the leading edge and the trailing edge.
In order to adjust, for example, the leading edge of a recording mark of 4T having an immediately previous space of 3T, the information recording control section 15 changes a recording parameter of 3Ts4Tm (for example, dTop). In order to adjust, for example, the trailing edge of a recording mark of 3T having an immediately subsequent space of 2T, the information recording control section 15 changes a recording parameter of 3Tm2Ts (for example, dTs).
A recording pattern generation section 11 generates an NRZI signal which is modulated by input information to be recorded. A recording compensation section 12 generates a recording pulse sequence in accordance with the NRZI signal based on the recording parameter changed by the information recording control section 15. A recording power setting section 14 sets recording powers including the peak power Pp, the bottom power Pbw and the like. A laser driving section 13 controls the laser light emitting operation of the optical head 2 in accordance with the recording pulse sequence and the recording powers set by the recording power setting section 14.
In this manner, test recording is performed on the information recording medium 1, and a recording pulse shape is controlled so as to decrease the edge shift amount. Thus, by the recording control method using the PRML system and space compensation of recording parameters, more appropriate recording marks and spaces can be formed.